Conventionally, card readers for reading magnetic data recorded in a card and recording magnetic data into a card are widely used. In the industry of financial organizations and the like in which card readers are used, traditionally so-called “skimming” is a serious issue in which a criminal installs a magnetic head at a card insertion part of a card reader so as to illegally obtain magnetic data of the card by use of the magnetic head. In order to avoid such a problem, proposed is a card reader equipped with a metal sensor for detecting a magnetic head for skimming (hereinafter, called a “skimming magnetic head”) having been installed at a card insertion part (for example, refer to Patent Document 1).
In a card reader described in Patent Document 1, a metal sensor includes a core made of a magnetic material, a couple of exciting coils wound on the core, and a detection coil. A value of voltage generated at both end parts of the detection coil fluctuates in accordance with a distance between the metal sensor and a foreign object containing a metallic material, for example, as shown in FIG. 6. Concretely to describe, if the metal sensor and the foreign object are positioned close to each other, the value of voltage becomes greater; and meanwhile if the metal sensor and the foreign object are positioned distant from each other, the value of voltage becomes small. Therefore, depending on whether or not the value of voltage of the detection coil exceeds a predetermined threshold value, it can be judged whether or not a skimming magnetic head has been installed at a card insertion part.
In the meantime, if an ambient temperature of the metal sensor fluctuates, a resistance and the like of the detection coil fluctuate accordingly so that the value of voltage of the detection coil fluctuates in accordance with the ambient temperature of the metal sensor, for example as a solid line curve shows in FIG. 7, even though the distance between the metal sensor and the foreign object stays constant. Therefore, if the threshold value to be compared with the value of voltage of the detection coil is constant and the ambient temperature of the metal sensor fluctuates, it cannot appropriately be judged whether or not a skimming magnetic head has been installed at the card insertion part.
Therefore, the card reader described in Patent Document 1 is provided with a temperature sensor for measuring an ambient temperature of the metal sensor; and a plurality of threshold values, each of which is in relation to an ambient temperature of the metal sensor, are stored in a control unit of the card reader. Moreover, in the card reader, a threshold value, to be compared with the value of voltage of the detection coil, is selected out of the plurality of threshold values stored in the control unit, in accordance with the ambient temperature of the metal sensor measured by use of the temperature sensor; and the selected threshold value is set up. As a result, even though the ambient temperature of the metal sensor fluctuates, the card reader can appropriately make a judgment on whether or not a skimming magnetic head has been installed at the card insertion part.